Due to the gradual depletion of fossil fuel and deterioration of environmental problems, many countries have paid much attention to the development and utilization of solar energy. Solar cells, which can convert solar energy into electricity, can be generally divided into the following groups: silicon solar cells, compound semiconductor solar cells, dye-sensitized solar cells and organic solar cells. Among all kinds of solar cells, monocrystalline silicon solar cells, polycrystalline silicon solar cells, and compound semiconductor solar cells are very expensive in terms of their material costs and are not eco-friendly, and organic solar cells have a reputation of low conversion efficiency and are therefore not suitable for large-scale commercial application. Accordingly, major research institutes are dedicated to developing alternative material and relevant technologies to reduce the fabrication costs and increase solar energy-to-electricity conversion efficiency.
Dye-sensitized solar cells have become the mainstream of the solar energy development due to their low cost, high efficiency, simple assembly and other advantages. Published by O'Regan and Grätzel in 1991, dye-sensitized solar cells use a porous film of nano-scale titanium dioxide (TiO2) as the working electrode, on which ruthenium bipyridine complexes (such as the N719) are adsorbed as a photosensitizer dye. Redox electrolyte comprising iodide/triodide (I−/I3−) is used as the electrolyte, and platinum-sputtered conductive glass is used as the counter electrode.
According to the operation principle of dye-sensitized solar cells, electrons of valence shell in the ruthenium dyes for light absorption are excited by light and jump to a higher energy level. Then the electrons are transmitted to the conductive layer of the nano-scale titanium dioxide semiconductor and led to an external circuit by the working electrode. The oxidized dyes obtaining electrons from the electrolyte of the cell are reduced to the initial state, and the circuit loop is completed after the electrolyte acquires electrons from the external circuit via the counter electrode.
Conventional electrolyte is organic solvent containing iodide/triodide; however, due to the low boiling point, volatility, high mobility of the organic solvent, this kind of electrolyte is not suitable for sealing of cells and long-term use. In addition, research found that liquid electrolyte can be replaced with polymer electrolyte in order to drastically increase the stability of solid-state dye-sensitized solar cells. However, the polymer electrolyte has low conductivity, and its electrode interface has poor infiltration, so the solar energy-to-electricity conversion efficiency of the solid-state batteries is far less than the liquid-state batteries. In recent years, ionic liquid used to replace organic solvent as the electrolyte in dye-sensitized solar cells has been studied extensively. Ionic liquid has high conductivity, low volatility, stable physical and chemical properties, wide range of working range, and high dielectric constant; therefore, the use of ionic liquid as the electrolyte in dye-sensitized solar cells overcomes the shortcomings of volatile liquid electrolyte. Grätzel and others utilized ionic liquid as the electrolyte of dye-sensitized solar cells and found that the solar energy-to-electricity conversion efficiency was raised up to 7% (with photon flux 100 mW/cm2); however, the high fluidity of the electrolyte is detrimental to battery sealing.
In recent years, the solar energy-to-electricity conversion efficiency of dye-sensitized solar cells with liquid electrolyte has been improved to 7-12%. one of main effects on the solar energy-to-electricity conversion performance of dye-sensitized solar cells is a stable and effective redox reaction which allows the stable and balanced existence of electrons and holes in different layers of the cells. Accordingly, how to improve the constitution of electrolyte material to enhance the solar energy-to-electricity conversion efficiency of dye-sensitized solar cells has become an urgent issue to researchers all over the world.
Many studies have been focused on the addition of additives into the electrolyte, such as 4-tert-butyl pyridine (4-TBP), benzimidazole, N-methyl benzimidazole (NMBI), to enhance the solar energy-to-electricity conversion efficiency (η %). Although the conventional electrolyte additives can increase the open circuit voltage (Voc) to enhance the solar energy-to-electricity conversion efficiency, the amount needed to be added is very high; furthermore, long-term use will result in crystallization in the cells, resulting the loss of their original functions and making the solar energy-to-electricity conversion efficiency instable. In addition, the conventional additives decrease the short circuit photocurrent density (Jsc), which causes the solar energy-to-electricity conversion efficiency cannot be optimized.
Therefore, it is the main objective of the present invention to discover an electrolyte additive that can be added to the electrolyte with a low concentration, and can be long-term used without resulting in crystallization in the cells and the loss of original functions. The additive also can provide an improvement in the short circuit current density and the solar energy-to-electricity conversion efficiency of solar cells.